The invention relates to an active aerial including an aerial element having a length which is small with respect to the wavelength associated with the operating frequency, and to an amplifier connected to the aerial element.
Such an aerial is disclosed in inter alia the article "Die transistorierte Empfangsantenne mit kapazitiv hochohmigem Verstarker als optimale Losung fur den Empfang niedriger Frequenz" by H. Lindenmeier, published in NTZ 27 (1974) no. 11, pages 411-417, and is used in mobile radio receivers, including car radios, and in what is known as aerial "arrays", in view of its small dimensions, the fact that it has such a large broad-band and the possibility to match the output impedance in a simple way to the characteristic impedance of the cable to a receiver.
Noise and distortion are unavoidable because of the presence of an amplifier, so that an optimum signal-to-noise ratio and a minimum intermodulation must be the aim when designing aerials of this type. Particularly when the aerial is used in the so-called aerial "arrays", it is furthermore important that the aerial transfer function is accurately defined and preferably independent of the frequency, while the influence of atmospherics must be suppressed to the best possible extent.
From the above-mentioned article it appears that the input impedance of the amplifier of the active aerial must be highly resistive.